With some dialects and speakers, it never is. With others, "tú" becomes "thú" in exactly the same environments that "sé" becomes "é", "sí" becomes "í" and "siad" becomes "iad". In other words, no one would ever say "tá é" or "tá thú", but they do say "feicim é" and "feicim thú" (althogh "feicim tú" is also correct). Finally, in some Texts (and for some modern speakers, esp. in Munster??) the choice between "tú" and "thú" is governed also by the last sound (vowel or consonant) of the word before, such as in these two variants from "Caoineadh Airt Uí Laoghaire":
"Mo ghrá thu go daingean!"
"Mo ghrá go daingean tu!"
Stair an Fhocail: séan
http://www.eskimo.com/~donncha/stair3.html
